1. Field of the Invention
The invention relates to a monitoring system for an intravenous feeding apparatus. In particular, the invention is directed to electronic circuitry which provides for the accurate detecting of the drop rate by using a dynamic reference threshold.
This application is related to a similar patent application filed with this application, by Carr et al.
2. Description of the Prior Art
It is common in medical practice to administer fluids to patients by means of an intravenous feeding apparatus. In more recent times, it has become more common to use sophisticated electro-mechanical intravenous delivery systems. As such, it has become necessary to count drops falling within the drip chamber, to both assure that the intravenous apparatus is functioning properly and also to control the drop rate or liquid flow rate.
Past intravenous monitoring systems have used drop detectors, in which a drop breaks a light beam projected onto a light sensor which in turn generates an electrical pulse which is directed to electrical processing circuitry. Campbell et al. U.S. Pat. No. 3,631,437, although not using optical detectors to detect liquid, does generate a series of electrical pulses by conductors embedded in the drip chamber which provide electrical input into logic circuitry that actuates an alarm when the drop rate frequency exceeds or falls below a specified rate.
Typical problems associated with optical detectors in intravenous monitoring systems are: the difficulty of detecting small droplets (1/60 ml. in volume) which only partially occlude the light beam; and the difficulty of detecting a steady stream of solution when the stream is generally too small (1 mm. in diameter) to obscure enough of the light beam.
A typical solution to the above-described problems is to increase the sensitivity of the signal processing circuitry. Such a solution presents its own particular problems when the walls of the drip chamber fog up by the build up of condensation. The build up of condensation results in a gradual decrease in light transmission through the drip chamber. Therefore, a typical droplet produces one signal in a clear drip chamber but a reduced signal in a fogged drip chamber. Other monitoring systems either ignore the effect of condensation or increase the gain of the system to operate in such conditions. A system that only increases gain must have a proportionally higher dynamic range.
Electronic circuitry which provides a dynamic reference threshold is sometimes used in comparator circuits to adjust the sensitivity of the threshold in response to the input. In typical circuits that provide a dynamic reference threshold, an input signal supplied by a signal generator or detector is directed to the inverting input of a comparator circuit, and the input signal is also directed to the dynamic reference threshold circuitry. The dynamic reference threshold circuitry reduces the input signal by using the voltage drop across a diode and directing this reduced input signal to the noninverting input of the comparator circuit. A capacitor is coupled to the dynamic reference circuitry between the diode and the comparator circuit so as to store a portion of the reduced input signal and to provide a residual signal when there is a sudden drop in the input signal. Therefore, when a sudden drop in the input signal occurs, the signal emitted by the dynamic reference threshold circuitry remains the same, while the input signal supplied to the comparator circuit by the signal generator or detector is reduced. The output of the comparator circuit changes when one input signal exceeds the other input signal. Examples of such circuits are illustrated by Kreda, U.S. Pat. No. 3,708,678, and Winebarger, U.S. Pat. No. 4,217,553.